1. Field of the Invention
This invention relates to terrain aided navigation, and more particularly to autonomous range-only terrain aided navigation, that generates range predictions based on a model of the radar altimeter on a grid aligned to a search space that varies dynamically over a moving measurement history and upon satisfaction of terrain sufficiency criteria correlates the range predictions to the measurement history using a terrain height database with a format that is independent of the flight characteristics.
2. Description of the Related Art
Terrain Contour Matching (TERCOM), first developed in the 1950's and patented under U.S. Pat. No. 3,328,795, uses radar altimeter measurements to compute a history of terrain heights and correlates that history with terrain heights from a database map. TERCOM has been used for cruise missiles. The main limitation of TERCOM is the labor-intensive preparation to tailor a set of terrain maps for each mission. The map height grid points, or cells, are aligned with the downtrack/crosstrack direction of a predetermined flight path and the spacing of the cells is the same as the spacing of the measurements with little angling or spacing discrepancy allowed (6:25-28, 14:7). The missile is constrained to straight flight at a certain speed while taking the measurements. A further constraint stems from the assumption that the radar altimeter provides the range to the terrain directly below the vehicle. In reality a conventional radar altimeter provides the range to the closest terrain in the radar beam called the “first return”. Conventional radar altimeters do not provide a bearing angle to the closest terrain. When this first return is not directly below the vehicle, it results in a terrain height that does not correspond to the terrain height associated with the vehicle's position in the database. This can cause false correlations. This further constrains TERCOM position corrections to be taken over terrain that is not likely to produce these false correlations, but at the same time has sufficient uniqueness to provide a good correlation. This requires detailed analysis of each mission and its associated terrain and limits the flexibility of mission planning. This work can be reduced by limiting the number of TERCOM position corrections and augmenting the navigation solution with other sensors, and eventually using GPS.
An improvement to TERCOM developed by John Hopkins University Applied Physics Laboratory (APL) used a reference map transform. At the mission planning and map generation stage, a first return radar altimeter model is run over the terrain at the planned clearance height, and the height associated with the first return is substituted into the cell corresponding to each possible horizontal vehicle position in the TERCOM map. These heights match the first return heights more closely. This resulted in a position correction with reduced errors and fewer false correlations. However, computing this reference map transformation involves a lot of pre-mission work and does not solve the problem that the mission is constrained to a particular clearance height, heading and speed.
U.S. Pat. No. 6,218,980 suggests performing a reference map transformation, similar to APL's reference map transform, with a radar altimeter model during flight as a function of current clearance height, applying this to a conventional TERCOM map. It is claimed that this allows for proper terrain correlation at high altitudes. Another improvement allowed for terrain correlation over a curved flight path. This approach still requires the cells to be given in downtrack/crosstrack coordinates and with a spacing corresponding to a particular vehicle speed, thus still requiring the generation of mission-specific maps before each mission.
U.S. Pat. No. 7,522,090 suggests a modification to the conventional TERCOM that allows for the use of a terrain height database that can be formatted independently of the flight path. A reference basket is calculated onboard from the horizontal position uncertainty, and sample points comprising the horizontal position and altitude sample (i.e. the difference between the vertical inertial altitude and the clearance altitude at the horizontal position) are correlated over the reference basket. The sample history continues to grow until a satisfactory correlation can be achieved. The reference basket is a set of cells in the terrain database and remains constant over the history.
Honeywell developed an interferometric radar sensor and associated algorithm called Precision Terrain Aided Navigation (PTAN), patented under U.S. Pat. No. 6,512,976. This has an advantage over the previous methods discussed above in that it provides more precise measurements, and in particular provides not only the range to the nearest terrain, but also bearing angles. Locating a terrain feature with an accuracy that is finer than the resolution of the terrain database allows for a more accurate position fix. A disadvantage is that the sensor and associated processing equipment and algorithms are very complex. PTAN requires three antennas with a significant special separation to get the full angular information, which limits its use to vehicles that are large enough to accommodate this spacing. Furthermore, PTAN requires expensive calibration of the boresight angles to a high accuracy.